THE centre of Sydney was thrown into chaos yesterday afternoon when all city train stations were closed down for more than five hours after a mysterious gas leak.

Tens of thousands of commuters were left stranded as the shutdown ground the CBD rail network to a halt and caused massive delays throughout the system.

It was not until 6pm that rail stations were reopened.

The City Circle line was shut down shortly after 1.30pm after rail workers noticed a strange smell at the busy Town Hall underground train station.

Within 20 minutes, Wynyard, Circular Quay, St James and Museum stations were evacuated and closed, shutting down eastern suburbs train services and clogging up the entire rail system.

NSW Fire Brigade Commissioner Greg Mullins said the gas leak permeated throughout the city’s rail network in a matter of minutes.

“It seemed to be moving with the trains,” he said. “In quick succession we had a number of triple-O calls.”

Police worked quickly to establish the gas was not the result of an act of terrorism, as emergency services rushed to implement a rehearsed evacuation plan of stations.

Fire brigade officers soon identified the smell as natural gas, but were unable to identify the source of the leak.

No one required medical treatment, but frustrated commuters were left wondering how, and when, they would get home.

City traffic ground to a halt and pavements were soon overflowing with grumbling workers, many of whom had no idea why the train stations were closed.

When Sydney’s main transport hub, Central Station, had to be closed an hour before the peak hour rush was due to begin, at 5pm, the city’s public transport system was on the verge of a meltdown.

Joy Willis had to take two buses from North Sydney to the city — normally a five-minute train ride — and join thousands of people in bus lines at Central.

“I know these things happen but they obviously cannot deal with a problem like this,” she said.

“I have no idea when I will get home.”

One older woman, too upset to give her name, shouted: “I am fed up, just fed up. It is absolutely disgraceful.”